Refuse a black mark (7)
'refuse' is the definition.
(I've seen this before)
'a black mark' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'mark' becomes 'stain' (synonyms).
(Other definitions for abstain that I've seen before include "Give backing to", "Cut it out!", "Refrain (from drink)", "Choose not to vote, or drink", "Formally decline to vote".)